1. Field of the Invention
The subject invention relates to a spark plug for an internal combustion engine, furnace, or the like, and more particularly to a spark plug having a high performance metal firing tip on its ground electrode.
2. Related Art
Within the field of spark plugs, there exists a continuing need to improve the high temperature oxidation resistance, erosion resistance and reduce the sparking voltage at the center and ground electrodes. To this end, various designs have been proposed using noble metal electrodes or, more commonly, noble metal firing tips applied to standard electrodes. Typically, the firing tip is formed as a pad or rivet or wire of a pure or alloyed precious metal composition, or of other high performance material composition, which is then welded to the end or side of the center electrode, ground electrode or both.
Platinum and iridium alloys are two of the noble metals most commonly used for spark plug firing tips. However, other alloy compositions have been used in various applications, including platinum-tungsten alloys, platinum-rhodium alloys. The use of additional alloy constituents such as yttrium and the like, have also been used with noble metal alloy to improve their operational performance.
While these and various other noble and high performance metal compositions typically provide acceptable spark plug performance, particularly with respect to controlling the spark performance and providing oxidation and spark erosion protection, current spark plugs which utilize noble metal tips have well-known performance limitations associated with the methods which are used to attach the noble metal components. Such attachment methods include the various forms of welding. Therefore, it is highly desirable to develop spark plugs having noble (or other high performance) metal firing tips which accomplish improved spark plug performance and reliability at lower cost and which can be readily affixed to the end of an electrode using known welding techniques. It is also highly desirable to develop methods of making spark plugs that will achieve these performance and reliability improvements using high speed production equipment of the type found in modern manufacturing plants.
A particular area of attention includes the manner in which a high performance metal firing tip is attached to the distal end of the ground electrode. Various techniques have been proposed, including seating the metal firing tip in a notch or pocket formed in the distal end of the ground electrode, as shown in U.S. Pat. No. 6,853,116 to Hori, et al., granted Feb. 8, 2005. A similar technique is shown in U.S. Pat. No. 4,700,103 in the name of Yamaguchi, et al., issued Oct. 13, 1987, as well as in U.S. Pat. No. 5,556,315 to Kagawa issued Sep. 17, 1996.
A more recent movement toward the use of high performance metal firing tips has been motivated by the goal of extending the serviceable life of a spark plug. Current expectations set the serviceable life of a spark plug fitted with a high performance metal firing tip beyond 100,000 miles of operation. As can be appreciated, stringent demands are placed upon the portions of the spark plug exposed to the combustion chamber. Accordingly, the manner in which the metal firing tips are attached to the base electrode components becomes especially important as a spark plug nears the end of its serviceable life. In particular, failure of the weld joint between the metal firing tip and the ground electrode can bring a premature end to the serviceable life of an otherwise high performance spark plug. The addition of sparking tips to the center and ground electrodes add steps to the assembly and manufacturing processes associated with spark plugs which utilize these features. Additionally, it is necessary to maintain precise control of the spark gap between the sparking surfaces located on the sparking tips, including maintaining precise control of the distances between the surfaces as well as their alignment relative to one another. Accordingly, metal firings tip and electrode configurations that facilitate the assembly process, including location and alignment of the sparking tips and surfaces on the center and ground electrodes, thereby lowering the cost of producing spark plugs with high performance metal sparking tips while maintaining the necessary spacing and alignment between them is also very important.
Extension of the spark plug service life, including the weld joints used to attach the high performance sparking tips to the center and ground electrodes, as well as the operating performance of the spark plugs which incorporate them are affected by the ability to remove heat from the sparking tips and electrodes during operation of the spark plug. Currently, copper cored nickel alloy center and ground electrodes are used to improve the thermal conductivity and ability to remove heat from the sparking tips. However, the effectiveness of such electrodes is directly related to the proximity of the high thermal conductivity core material to the sparking tip. The more closely the thermally conductive core material can be placed to the sparking tip, the more heat that can be removed from the sparking tip. Accordingly, the development of sparking tip configurations that permit control of the spacing between the sparking tip and the core material are desirable.
Accordingly, improvements in the manner in which a high performance metal firing tip is attached to the ground electrode are highly desirable within the industry and are useful to improve the performance and extend the serviceable life of a spark plugs of this type.